As investors waited for the outcome of the last US Federal Reserve meeting of the year, Centrica shares flared up.

The British Gas owner added 5.6p to 328.7p after unveiling the £420m sale of its Texas gas-fired power stations to Blackstone, with the proceeds destined to be handed back to investors via a continuation of its share buyback programme in 2014. Analyst Angelos Anastasiou at Whitman Howard said:

Gas-fired generation has not been particularly profitable in recent times in Texas, whereas the continuation of the buy-back should be moderately earnings enhancing; so we see this as good news.

Overall the FTSE 100 edged up 5.89 points to 6492.08, coming off its best levels as nervousness increased ahead of the Fed announcement later. Investors have been fretting that Ben Bernanke might signal a slowdown in the Fed's $85bn a month bond buying programme, although most believe it is more likely that this will be delayed until the new year.

UK unemployment fell back sharply in October to 7.4%, bringing it ever closer to the 7% staging post"identified by Bank of England governor Mark Carney as a benchmark to review UK monetary policy.

The main focus though remains very much on whether the Fed will be bold enough to start the long awaited tapering program later this evening as Ben Bernanke chairs his very last press conference as Fed Chairman.

Supermarkets came under pressure again following Tuesday's news that the big four were losing market share to discounters. J Sainsbury was the biggest faller in the FTSE 100, 13.5p lower at 365.1p, while Tesco was down 5.45p at 319.05p and Morrisons lost 3.5p to 253.2p. Deutsche Bank said:

The Big 4 [Wal Mart-owned Asda is the fourth] have entered a period of structurally lower growth, due to a high level of consolidation and increased competition. We maintain our sell rating on Morrisons (target price 250p), our hold rating on Sainsbury's (target price 410p) and our buy rating on Tesco (target price 386p)

Still with retail, Marks & Spencer fell 10.8p to 441.5p, after downbeat comments from UBS and a price target cut from HSBC. There was also talk that its non-food sales were £30m below budget last week, while an increasing number of discounts are likely to hit margins.

Debenhams dropped 3.55p to 78.45p as UBS cut its profit forecast by 11% for 2014 and 10% for 2015.

Elsewhere United Utilities was 17.5p lower at 650.5p after the shares went ex-dividend, while Petrofac fell another 21p to £10.89 and Amec dipped 6p to £10.42 following Tuesday's warning from oil services specialist CGG.

But Rexam rose 9.2p to 497.3p after a positive note on the packaging sector from Bank of America Merrill Lynch.

Tullow Oil was steady at 836.5p despite disappointing results from its Paon-2A well at the Autruche field in the Ivory Coast. The well encountered water and will be plugged and abandoned. Tullow's exploration director Angus McCoss said:

The well did not encounter oil at this location, reducing the upside volume potential associated with the Autruche field.

Oriel Securities kept its add recommendation on Tullow, saying:

We see the announcement as slight negative for Tullow. At $8 a barrel of oil equivalent and 50% chance of success, this would imply a negative impact of about $344m net to Tullow (or about 3% on the current market capitalisation). However, as stated before, we don't believe markets currently assign too much value on future exploration success. Therefore we expect an impact significantly lower than 3%, likely in the 1% range (especially that the company was carried for half of its costs on this well).

Among the mid caps, iron ore miner Ferrexpo fell back after its two largest investors unveiled plans to sell some of their shares.

Ferrexpo ended 1.4p lower at 182.6p as chief executive Kostyantin Zhevago and investment group BXR said they would trim their shareholdings to help meet new rules for the amount of a company's shares which available on the market.

Finally Vectura added 16.25p to 137.25p after it announced its partner Sandoz had received Danish approval for the sale of an inhaled lung drug, AirFluSal Forspiro, that analysts say could be a generic copy of GlaxoSmithKline's blockbuster drug Advair. Investec analyst Nicholas Keher said:

We believe this is a generic of Seretide [Advair] and see it as a material value driver for Vectura. Un-risked, this now adds 22p to our target price. However, as previously highlighted, given time and assuming approvals are achieved globally for the portfolio, we think this stock could be worth 270p on a two year view. Buy.